Efficient operation of modern chlor-alkali membrane cells requires that many contaminants in the anolyte brine be removed or neutralized before electrolysis starts. Early in the development of these cells, it was identified that concentrations of alkaline earth metals, such as calcium and magnesium, in the brine should be held to levels below about 50 parts per billion (ppb) and preferably below about 10 ppb to achieve long-term, high efficiency operation. Where the brine used is derived from a reconstituted depicted brine anolyte, other brine contaminants found to negatively affect cell performance are sulfates, chlorates and hypochlorites which plug or otherwise degrade the membrane during electrolysis.
Normally, treatment to remove these contaminants comprises admixing the reconstituted brine first with alkali metal carbonate to precipitate dissolved calcium and then with an alkali metal hydroxide to precipitate the magnesium. Where present, excess sulfate ions can be removed by adding barium chloride thereto or by purging part of the brine stream. Occasionally, flocculants, such as aluminum or ferric chloride, are added to reduce the settling time required to remove these precipitates. Such a process can produce the saturated brines having residual impurity contents as shown in Table I:
TABLE I ______________________________________ Impurity Level ______________________________________ Calcium (ppm) 1 Magnesium (ppm) 0.1 Sulfate (gpl) 10 Fe (ppm) 0.1 ______________________________________
at a pH of about 11. Such a pH value is generally too high for membrane cell use, so a mineral acid, usually the halide acid, is added to lower the pH value, preferably to between about 2 and about 4. Further, by so doing, any excess carbonate present is removed along with any residual hypohalite or halate ions remaining after dechlorination.
While such brines are suitable for use in mercury and diaphragm cells, the calcium and magnesium values, in particular, are still too high for use in many cells using high performance permselective cationic membranes. For such use, an additional treatment in which the brine at a pH of about 8.0 is admixed with an ion exchange resin to effectively lower the calcium and magnesium values to levels approaching 10 ppb (0.010 ppm) is applied. Techniques for performing these operations are well known in the art.
Reconstituted brine also contains a small amount of aluminum, both in ionic and colloidal form, which is normally present at a level of between about 0.1 and about 2.5 parts per million (ppm), and silica, which is normally present at a level of between about 0.1 and about 20 ppm. Most recently, it has been found that these impurities also have a capability to damage the membrane and significantly affect its sodium and water transport properties. Further, when present at the same time, aluminum and silica, at a pH above about 3.5, form a stable, nonionic colloidal complex which proves to be quite difficult to remove. While techniques, such as ultrafiltration, have some utility for removing uncomplexed silica, it is found that they are not particularly effective in either breaking the complex or removing the aluminum.
What is needed is a technique for breaking this complex and converting the aluminum and silica therein to forms in which they may be effectively and efficiently removed from said brine.